Air purifiers are a great way of removing smells and odours from the home. The effectiveness of removing odours depends on the filter used by the air purifier.
Air purifiers are a great way of removing airborne allergens. They are very effective at removing pollen particles, dust and animal dander from the air. They will not by any means cure allergies or asthma but will be great for calming them down.
The air purifier should be placed in the room you spend most time in. For example your bedroom or living room. The air purifier will slowly purify most of your house, it will just take a while, which is why you should start with the room you spend most time with.
If is recommended by most to run the air purifier for at least 12 hours a day everyday. Of course this does and will not work for everyone, so we would recommend turning the air purifier on when you wake up. Let it run whilst you are out of the house at work or running errands.
Then once you come back home from work turn it off and you air should be purified.
The short answer is yes, air purifier are loud. Air purifiers exchange the air in the room, filter it through and release the air. That process is naturally loud, you can expect it to be around the same noise level as a desk fan.